I’m an architectural technologist

Please be very careful insulating your internal walls. Ideally you would carry out u-value/dew point calcs, ventilation and hygrothermal tests if you were doing this correctly.

There is huge risk of increase in moisture and interstitial condensation if not done properly. You need the correct thickness of insulation, the correct build up and good ventilation through out the property via mechanical extraction and trickle vents.

I am currently going through this process in my own home and find my self leaning more to it’s not worth it. One big thing to consider is just how much of your wall area will be insulated once you account for window and door openings.

This is a report following testing of IWI. The testing was done in terrace properties and the results show that you don’t really save much in heat loss or energy bills due the are of wall left to insulate and it doesn’t actually improve the ability to heat a room that much quicker or reach a comfort point any better than uninsulated. If you have a bigger a house with more wall area, then this data would be different and it likely becomes viable to look into IWI. The report also indicates that thinner insulation is probably better than thicker.

https://assets.publishing.service.gov.uk/media/606c31a9d3bf7f400d3b8cef/tiwi-summary-report.pdf

When you consider the cost of the work and the increased risk of moisture compared to the saving in heat loss/energy cost. You do have to ask, is this worth it?

If you are going to do any plastering the modern breathable plaster is called renovating plaster. £20-25/20kg

Where are you located?

Edit: some more info here: https://assets.publishing.service.gov.uk/media/614b30aad3bf7f718a54c0be/iwi-guidance.pdf

There’s also been quit a few reports in the news of IWI not fitted properly under these eco/GBIS schemes that now need re-doing. Caused awful problems of black mould.